The general structure of a torque sensor configured to detect torque using a shaft having a magnetostrictive region is shown in FIG. 14. A shaft 1 that is subjected to torque is supported in a housing 7, by way of a bearing 6, and magnetostrictive regions V/W are formed around the entire circumference) (360° of a portion of the surface of the shaft 1. Furthermore, coils X/Y are disposed at positions near the respective outer circumferences of the magnetostrictive regions V/W, within the housing 7. Generally, for the magnetostrictive regions V/W, magnetic material is formed on the side face of the shaft 1, in helical striped patterns inclined in mutually opposite directions with respect to the axial direction (which is to say, in chevrons), as shown in the figures (in other words, the coatings or protrusions of the magnetic substance are formed as multiple helical lines). As shown in the figure, the housing 7 is also provided with an amplifier board 8, a signal line connector 9 and the like.
When a torque acts on the shaft 1, tensile stress and compressive stress are generated in the respective magnetostrictive regions V/W and, consequently, the magnetic permeability of the magnetostrictive regions V/W respectively increases or decreases due to mutually opposite magnetostrictive effects. Induced electromotive forces are generated in the coils X and Y based on the change in the magnetic permeability, and therefore a voltage output that is proportional to the magnitude of the torque can be obtained by performing DC conversion and differentially amplifying the two.
Patent Literature 1, below, discloses a shaft having excellent torque detection characteristics, which can be used for a torque sensor of the type described above. That is to say, the surface of the shaft is provided with a magnetostrictive region including a coating of metallic glass (amorphous alloy), the coating being formed on the surface of the shaft by thermal spraying, with a method in which the flame is rapidly cooled. Thermal spraying in which the flame is rapidly cooled refers to a thermal spraying method in which a flame including a metal powder is sprayed, melting the metal powder, and the flame is cooled from the outside by a cooling gas, before it reaches the shaft surface.
When the shaft described in Patent Literature 1 is used for torque sensor shafts, the torque detection characteristics are improved for the following reasons. That is to say:
a) because the amorphous alloy has high magnetic permeability and high magnetostriction, a torque sensor using the shaft having the metallic glass in the magnetostrictive region will have high torque detection sensitivity;
b) since the metallic glass coating is formed on the surface of the shaft by thermal spraying, the adhesion between this coating and the shaft will be high, so that the hysteresis will be lower than in cases where this coating is fixed with an adhesive or the like; and
c) if this coating is formed by thermal spraying, because the coating is laminated onto, and adhered with, the shaft simultaneously with amorphization, the magnetostrictive region will be formed easily and quickly. If a method is adopted in which a flame including a metal powder is sprayed so as to first melt the metal powder and so as to be rapidly cooled, the adhesion of the metallic glass coating to the shaft surface will be particularly high, which is advantageous in view of b), above.